Offshore platforms for the exploitation of undersea petroleum and natural gas deposits typically support production risers that extend to the platform from one or more wellheads or structures on the seabed. In deep water applications, floating platforms (such as spars, tension leg platforms, extended draft platforms and semi-submersible platforms) are typically used. These platforms are subject to motion due to wind, waves and currents. The risers employed with such platforms must therefore be tensioned to permit the platform to move relative to the risers. Riser tension must also be maintained so that the riser does not buckle under its own weight. The tensioning mechanism must accordingly exert a substantially continuous tension force to the riser within a well-defined range.
Hydro-pneumatic tensioner systems are one form of riser tensioning mechanism typically used to support risers known as “Top Tensioned Risers” on various platforms. A plurality of passive hydraulic cylinders with pneumatic accumulators is connected between the platform and the riser to provide and maintain the necessary riser tension. Platform responses to the above mentioned environmental conditions, mainly heave and horizontal motions, create changes in riser length relative to the platform, causing the tensioning cylinders to stroke in and out. The spring effect resulting from the gas compression or expansion during riser stroke partially isolates the riser from the low heave platform motions. For horizontal (or drift off) motions, the compression of gas causes a load variation on the tensioning cylinders that is similar to that of the heave motions.
Hydraulic cylinders constituting such hydro-pneumatic tensioning systems comprise pistons in which the piston rods are at least indirectly connected to the riser so that the pressure induced movements of the piston relative to its cylinder results in the desired riser tensioning.
Such hydro-pneumatic tensioner systems are presently produced in a variety of dimensions/sizes, each corresponding to a certain load capacity (further detailed below). Examples of typical state of the art riser tensioner systems are described in U.S. Pat. Nos. 4,886,397, 3,902,319, GB 2,109,036 and U.S. Pat. No. 5,846,028 A.
The above prior art tensioner systems all constitute systems designed for a particular load capacity. There is normally space for just one tensioning system size on a drilling rig/floating platform, hence setting a clear limitation of the available load capacity range. During the design of a rig, the designer, having knowledge of these prior art systems, takes into account which type of operation the rig must perform, and then chooses the tensioning systems based on the maximum requirement for that particular rig. The criteria for choice of rig, and thereby tensioning size, may be sea depths, mud weight, riser type, and task. The latter normally varies from e.g., wildcat drilling requiring large dimensional systems to e.g., workover or production testing requiring significantly less riser tensioning capacity. The need for riser tensioning may be as little as 10% of maximum capacity of the installed riser tensioner. In these cases, the largest and heaviest tensioner systems are generally unsuitable since the relatively large load variations introduce risks such as fatigue on the wellhead and/or on the riser.
FIGS. 2, A and B shows prior art drawings of two different cylinders presently used in the industry. The effective cross section of the cylinder piston 2, being composed of a piston rod 2′ and a piston head 2″ and slidingly journaled within the cylinder 1, determines, in addition to the net pressure difference across the piston head 2″, the total force (F=pA) exerted onto the cylinder piston 2 and thereby on the riser in question (not shown). A gas pressure source and a suitable high-pressure pneumatic accumulator 5 allow at least some control of the piston movement induced cylinder pressure.
To provide correct capacity for the various rig requirements during design, two alternative arrangements have primarily been chosen; single acting (FIG. 2 A) and double acting (FIG. 2 B). Each of these is described below.
Single acting: The simplest way of arranging the hydraulic cylinder in riser tensioning systems is as a so-called single acting cylinder, hereinafter referred to as a “plunger”. In a plunger, the piston head 2″ is normally seal free and comprises several piston perforations 6 perforating the piston head 2″, thereby equalizing the pressure (P) above and below the piston head 2. When the piston 2 is displaced within the cylinder 1, the cylinder fluid located therein flows through perforations 6. The effective cross section (A) of the piston 2 is therefore equal to the effective radial cross section of the piston rod 2′. The required fluid pressure within the cylinder 1 is provided by a single high-pressure accumulator 5 being in fluid communication with the cylinder 1 via a high pressure conduit 8. The high pressure accumulator 5 is further typically divided into a high pressure gas end 5′ and a high pressure fluid end 5″ by a floating piston 7, where the high pressure conduit 8 is connected to the high pressure fluid end 5″. To provide the necessary pressure on the high pressure gas end 5′, and thereby causing the desired compression of the fluid in the high pressure fluid end 5″ by the resulting translational movement of the floating piston 7, a gas bank (not shown) is connected in fluid communication via a high pressure gas conduit 9 to the high pressure gas end 5′.
Dual acting: FIG. 2 B shows an alternative hydraulic cylinder in a prior art riser tensioning system; a dual acting cylinder. As for the plunger, the dual acting system comprises a compressed cylinder 1 with a piston 2,2′,2″, a high-pressure accumulator 5,5′,5″, a floating piston 7, a high pressure conduit 8, and a high pressure gas conduit 9. In contrast to the plunger arrangement, however, the piston head 2″ in a dual acting cylinder forms a fluid tight separation with the inside walls of the cylinder 1, thereby effectively dividing the cylinder 1 into two mutually fluid tight chambers; a first cylinder chamber 1′ and a second cylinder chamber 1″. The separation may be achieved by arranging one or more piston seals 20 between the inner wall of the cylinder and the circumference of the piston head 2″. To be able to control the pressure in the first cylinder chamber 1′, i.e., on the piston rod 2′ side, a pressure source, such as a low pressure accumulator 10, is connected in fluid communication with the first cylinder chamber 1′ via a low pressure conduit 11. In FIG. 2 B, this low pressure accumulator 10 is illustrated as a partly fluid filled container into which an open end of a low pressure fluid conduit 11 is inserted. The pressure formed in the first cylinder chamber 1′ must be low enough to avoid that the pressure force set up in the second cylinder chamber 1″, due to the above mentioned high-pressure accumulator 5,5′,5″, is not significantly counteracted. For further details about dual acting systems, see, for example, US 2008/0304916 A1.
Tensioning systems applying these two types of hydraulic cylinders provide tensioning of risers that covers the two capacity requirements mentioned above. More specifically, the plunger type and the dual acting type may successfully be applied for low capacity requirements such as workover or production testing and high capacity requirements such as wildcat drilling. As indicated previously, there is a need, however, for a riser tensioner system which may handle the above mentioned variations of the drilling rig requirements without introducing any significant operational risks such as fatigue on the wellhead and/or the riser.
Various tensioner systems have been disclosed which are able to handle capacity variations. One example is U.S. Pat. No. 4,362,438 which describes a tensioner system having at least two hydraulic cylinders connected to the riser in question. The purpose of the solution described in U.S. Pat. No. 4,362,438 is to redistribute total pressure among any remaining functional hydraulic cylinders to maintain constant riser tensioning in case of undesired pressure changes, such as the failure in one or more cylinders. It therefore does not provide a system that allows a change between a large, high capacity, system and a small, low capacity system. Another example is U.S. Pat. No. 4,351,261 which describes a heave compensation system that adjusts to various operations with different requirements for load equalization by means of switching between several high pressure accumulators. This solution also does not allow a change between a large system having high capacity and a small system having low capacity. All the above mentioned publications therefore disclose solutions to problems which differ significantly from the problem of the present invention, i.e., to handle large capacity variations due to predictable changes in drilling rig requirements.